A known conventional optical particulate matter sorter has a unit for ejecting air to separate grains, such as cereal grains or resin pellets, into non-defective grains (desired grains) and defective grains (unwanted grains) or remove foreign matters from the grains.
The particulate matter sorter of this type separates grains by activating the above-described unit in response to a signal of detection of a defective grain or the like to eject air to blow off the defective grain when the grain is falling from an end part of a conveyor path along a predetermined trajectory.
The particulate matter sorter ejects air to blow off defective grains or the like in a large amount of grains continuously falling. To precisely blow off the defective grains or the like without blowing off the other grains, the air ejecting nozzle has to have a valve with a high responsivity.
In Patent Document 1, a piezoelectric air valve is described that uses a piezoelectric element to quickly open and close the valve. The piezoelectric air valve is provided with a displacement enlarging mechanism that enlarges a small displacement of the piezoelectric element by the principle of leverage.
The piezoelectric air valve has a higher responsivity at the time of opening and closing of the valve than the conventional electromagnetic valve, and therefore, the optical particulate matter sorter provided with the air ejecting nozzle incorporating the piezoelectric air valve can precisely blow off defective grains or the like and is less likely to blow off non-defective grains or the like.
However, since the piezoelectric air valve moves a valve body via the displacement enlarging mechanism, the optical particulate matter sorter has a problem that, in case of long gas ejection time, the valve body vibrates and the amount of air ejected from the nozzle changes, and stable separation cannot be achieved.
In Patent Document 2, a piezoelectric valve is described that applies a two-level driving voltage to a piezoelectric element to prevent the variation of the amount of gas ejected from a gas discharge channel when the valve is opened. The piezoelectric valve applies a first level voltage to the piezoelectric element at the time to drive the valve open, and applies a second level voltage higher than the first level voltage to the piezoelectric element at the time to prevent the vibration of the valve body caused by the valve opening. With such a configuration, the variation of the amount of gas ejected from the gas discharge channel after valve opening can be prevented, so that the gas can be stably supplied even when the gas is ejected for a long time.
However, the piezoelectric valve has a problem that it has a significantly low responsivity at the time of valve opening compared with the case where a single-level rectangular driving voltage is applied to the piezoelectric element.
In addition, the piezoelectric valve has another problem that it is expensive because the driving device has a complicated circuit configuration compared with the case where a single-level driving voltage is applied to the piezoelectric element.